Carboxylic acids are important chemicals of commerce. They appear as desired or contaminating constituents of a wide range of aqueous process streams. There are many instances where it is important to remove and/or recover carboxylic acids from aqueous solutions. Examples include product recovery in the manufacture of chemicals from biomass by fermentation and both product recovery and processing of aqueous wastes in the petrochemical, chemical, pulp and paper industries. There are also applications in other industries based upon biological materials, such as corn wet milling and processing of grains and food oils. Acids commonly of interest are acetic, formic, succinic, adipic, fumaric, maleic, lactic, malic and citric acids. There is strong industrial interest in lactic acid as a precursor to biodegradable plastics.
Present separation techniques for removal and recovery of carboxylic acids are energy-intensive and therefore expensive. Some, such as the classical method for recovery of citric and lactic acids by precipitation of calcium salts, also consume large volumes of chemicals (e.g., sulfuric acid and lime) and create large volumes of waste (e.g., calcium sulfate). Recovery technology using reversible chemical complexation with agents such as polymeric sorbents with amine functionalities can reduce energy consumption substantially. If an appropriate method of regeneration, allowing recovery and reuse of all agents, is utilized, such processes can also avoid production of waste salts and net consumption of chemical agents.
Previous researchers (Kertes and King, 1986; Tamada, et al., 1990; Yabannavar and Wang, 1987, 1991; Yang, et al., 1987; Garcia and King, 1989) have shown that extraction and adsorption by reversible chemical complexation are effective for recovery of carboxylic acids from dilute aqueous solutions. Amine-based extractants and adsorbents sustain uptake capacity for carboxylic acids from solutions at pH above the pK.sub.a1 of the acid, where the acid exists mostly as the carboxylate anion. However, these strongly basic complexing agents require correspondingly strong methods for regeneration (Tung and King, 1992, 1994). In this connection, it has been proposed in the art to add carbon dioxide to the extraction zone in an extraction process to provide carbonic acid to push the acid equilibrium in the direction of the free acid form, to enhance acid recovery (Baniel, Eyal, et al., 1996).
One approach to regenerating strongly basic extractants and adsorbents is leaching with an aqueous solution of a strong base (e.g., NaOH or Ca(OH).sub.2) to form the carboxylate salt. A strong acid (e.g., H.sub.2 SO.sub.4) must then be added to liberate the carboxylic acid product. This approach necessarily consumes chemicals and produces a waste salt by-product.
A second approach involves leaching with an aqueous solution of a volatile base, such as trimethylamine (TMA) (Poole and King, 1991, 1995). The resulting trimethylammonium carboxylate can be decomposed thermally, yielding acid product and recovering the TMA, making it available to recycle. For slightly soluble acids, such as succinic and fumaric, partial evaporation of the aqueous trimethylammonium carboxylate solution results in precipitation of the acid product (Poole and King, 1991). However, for lactic acid, which is highly soluble in water, the thermal decomposition of trimethylammonium lactate is incomplete (Poole and King, 1991), leaving about 0.6 mols TMA/mol lactic acid under the conditions used.
Ion exchange and adsorption have also been employed in carboxylic acid recovery schemes. U.S. Pat. No. 4,720,579 to Kulprathipanja discloses the use of styrene-divinylbenzene resins to adsorb citric acid with regeneration by water or by a mixture of acetone and water. Similarly, U.S. Pat. No. 4,323,702 to Kawabata discloses the use of adsorbents containing pyridyl functional groups combined with regeneration by leaching with an organic solvent such as an alcohol or a ketone. U.S. Pat. No. 4,924,027 to Kulprathipanja and Strong discloses adsorption of citric acid by adsorbents containing tertiary amine or pyridyl functionalities (including Bio-Rad AG3-X4A and AG4-X4), with regeneration using an aqueous solution of sodium, potassium or ammonium hydroxide, yielding the respective sodium, potassium or ammonium citrate. Treatment of these citrates with a strong acid would yield the free citric acid form, but again consumes an acid and a base and produces a waste salt stream. In each of these solutions the citric acid is adsorbed from an aqueous solution below the pK.sub.a1 of citric acid.
As can be seen from this description of background, various methods used heretofore to recover carboxylic acids have presented limitations and thus offer opportunities for improvement. It is accordingly a general objective of the invention to provide an efficient process for the recovery of carboxylic acids from aqueous solutions which neither consumes large amounts of chemicals nor generates waste chemical streams.